Vehicular air conditioning systems typically employ an air-cooled condenser for cooling and condensing the air conditioning refrigerant. An electric fan is used to enhance the flow of air across the condenser. Various methods for controlling the fan speed are known in the art. For example, U.S. Pat. No. 4,590,772 discloses a vehicular air conditioning system capable of adjusting draft volume across the radiator and condenser in response to air conditioner operating conditions. U.S. Pat. No. 5,285,650 discloses a controller which automatically turns off a condenser electric fan as the speed of the associated automobile surpasses a critical speed, or turns on the fan as the speed of the automobile drops below the critical speed. These references require relatively complicated control circuits and cabling external to the condenser fan motor.
Condenser fan speed controllers have also been used in refrigeration systems. For example, U.S. Pat. No. 3,122,895 discloses a condenser fan controller for a refrigeration system wherein a clutch disconnects the fan propeller shaft from a motor driven shaft when the condenser pressure drops below a certain value. U.S. Pat. No. 3,293,876 discloses a refrigeration system including control arrangement for regulating power input to the fan drive motor of an air-cooled condenser in response to the refrigeration load. U.S. Pat. No. 3,613,391 discloses a pressure-responsive motor speed control for maintaining minimum condenser pressure in a refrigeration system under varying ambient temperature and refrigeration load conditions.
Another type of fan speed controller used in vehicular air conditioning systems includes an automobile computer programmed to trigger a separate relay. The relay controls the flow of energy to a two-speed electric motor equipped with two brush sets and two commutator sets for low and high-speed rotation. The controller employs power resistors and regulators for varying motor speed. This design, however, has several disadvantages. First, the design is prone to failures leading to ineffective or unsafe conditions. For example, a computer failure may prevent the computer from commanding the relay to configure the fan for high-speed rotation at high condenser pressure or temperature. Also, controller reliability is lower due to the extra cabling and connections required between the car computer, relay and fan motor. Furthermore, the design is inefficient due to energy losses incurred by the power resistors and regulators. Finally, the design's two brush sets and two commutator sets increase the physical dimensions and weight of the motor.
Alternating-current motors in which the windings can be reconfigured by switches to vary the speed of rotation have been used. For example, U.S. Pat. No. 841,609 discloses a multi-speed alternating-current motor provided with windings which can be connected in various configurations. This alternating-current motor, however, cannot be used in direct-current applications such as vehicular air-conditioning systems.
Accordingly, it would be desirable to provide a safe and effective way to control the speed of a direct-current electric motor. It would also be useful to provide a direct-current electric motor with an integral (i.e., built-in) speed rotation switch directly activated by a vehicle sensed parameter signal, and a two-speed direct-current electric motor with a high-speed rotation switch which changes the motor speed by reconfiguring the motor windings during operation. It would also be desirable to provide a two-speed direct-current electric motor which drives a fan for cooling a vehicle air conditioning condenser based upon a sensed vehicle parameter such as the condenser pressure or temperature.